Degradable material assisted diversion or isolation

ABSTRACT

A method for well treatment by forming a temporary plug in a fracture, a perforation, a wellbore, or more than one of these locations, in a well penetrating a subterranean formation is provided, in which the method of well treatment includes: injecting a slurry comprising a degradable material, allowing the degradable material to form a plug in a perforation, a fracture, or a wellbore in a well penetrating a formation; performing a downhole operation; and allowing the degradable material to degrade after a selected time such that the plug disappears.

This application is a divisional application of U.S. patent applicationSer. No. 11/294,983, filed Dec. 5, 2005.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 11/156,966, entitled “Degradable Fiber Systems ForStimulation,” filed Jun. 20, 2005, hereby incorporated in its entirety.This application is also a Continuation-in-Part of U.S. patentapplication Ser. No. 11/206,898, entitled “Methods For Controlling FluidLoss,” filed Aug. 18, 2005, hereby incorporated in its entirety, whichclaimed the benefit of U.S. Provisional Application 60/606,270, filedSep. 1, 2004. The invention relates to stimulation of wells penetratingsubterranean formations. More particularly, it relates to fracturing andpost-job protection of hydraulic fractures.

BACKGROUND OF THE INVENTION

Hydrocarbons (oil, condensate, and gas) are typically produced fromwells that are drilled into the formations containing them. For avariety of reasons, such as inherently low permeability of thereservoirs or damage to the formation caused by drilling and completionof the well, the flow of hydrocarbons into the well is undesirably low.In this case, the well is “stimulated,” for example using hydraulicfracturing, chemical (usually acid) stimulation, or a combination of thetwo (called acid fracturing or fracture acidizing).

Hydraulic fracturing involves injecting fluids into a formation at highpressures and rates such that the reservoir rock fails and forms afracture (or fracture network). Proppants are typically injected infracturing fluids after the pad to hold the fracture(s) open after thepressures are released. In chemical (acid) stimulation treatments, flowcapacity is improved by dissolving materials in the formation.

In hydraulic and acid fracturing, a first, viscous fluid called a “pad”is typically injected into the formation to initiate and propagate thefracture. This is followed by a second fluid that contains a proppant tokeep the fracture open after the pumping pressure is released. Granularproppant materials may include sand, ceramic beads, or other materials.In “acid” fracturing, the second fluid contains an acid or otherchemical such as a chelating agent that can dissolve part of the rock,causing irregular etching of the fracture face and removal of some ofthe mineral matter, resulting in the fracture not completely closingwhen the pumping is stopped. Occasionally, hydraulic fracturing is donewithout a highly viscosified fluid (i.e., slick water) to minimize thedamage caused by polymers or the cost of other viscosifiers.

When multiple hydrocarbon-bearing zones are stimulated by hydraulicfracturing or chemical stimulation, it is desirable to treat themultiple zones in multiple stages. In multiple zone fracturing, a firstpay zone is fractured. Then, the fracturing fluid is diverted to thenext stage to fracture the next pay zone. The process is repeated untilall pay zones are fractured. Alternatively, several pay zones may befractured at one time, if they are closely located with similarproperties. Diversion may be achieved with various means. The commonlyused methods for stress/pressure diversion in multiple fracturing stagesare as follows.

The first method is the Bridge Plug technique. For example, the operatorperforates, then fractures, then sets a bridge plug, and then repeatsthis process as necessary. This approach ensures 100% positive zoneisolation by setting a packer between fractured and targeted zones.However, this approach is extremely costly. The costs come fromextensive wireline service intervention, which requires additional timeto perforate and to set and then retrieve the packer from the wellborefor each pay zone before and after a fracturing treatment. In addition,packer retrieval is sometimes risky.

The second method is the Flow Through Composite Bridge Plug (FTCBP)approach, which is a modification of the Bridge Plug. FTCBP works as aBridge Plug when there is higher pressure above it, such as duringsubsequent fracturing treatment. However, when the pressure is higherbelow the plug, such as when flowing the well back, the FTCBP lets fluidflow from below through the plug. Use of the FTCBP technique allows allpreceding fractured zones to flow during completion of the well. Thismethod has two advantages. First, it considerably reduces the shut-intime by flowing each fracture back early. Second, all previously treatedzones help to clean up each new treatment. After a well is completed,the FTCBP can be drilled out easily or can be left in the well. Thistechnique has proven to be a reliable tool that increases production.The main disadvantage is the cost and time needed to set the plug.

The third approach is the Sand Plugs technique. This is similar to theBridge Plug techniques except that sand plugs are used instead of tools.The main idea is to fracture several pay zones sequentially viadifferent perforation sets and set a sand plug at the end of eachtreatment stage to prevent flow beyond the plug, and thus divert thestress field for successive stages. This method substantially reducestime and costs because it requires no packer retrieval. However, due toinitial in-situ stress variations, not all zones may be fractured.Furthermore, the proppant placement requires loading the wellbore withproppant, which may result in low efficiency of the treatment.

The fourth method is the Limited Entry approach, which is a simplifiedtechnique that does not require loading the wellbore with sand. Thismakes the method more affordable. The method is used, for example, incombination with ball sealers to plug the stages, or by having differingnumbers of perforations for the different stages. The limited entrymethod basically relies on creating an artificial pressure drop across acalculated number of perforations. From the number of perforations, thesize of the perforations, and the injection rate, the pressure drop iscalculated. This pressure differential is then adjusted through thenumber of perforations to create a designated pressure on the formationside of the perforations equal to the fracturing pressure. Knowing theexact fracturing pressure of each sand layer is an essential portion ofthe limited entry technique. In an infill-drilling program within astratigraphic pay, the pressure of any given sand can vary considerably.Acquiring reliable pressure data involves testing each zone, adding timeand cost to the completion. Without knowing the exact data, a treatmentmay result in little or no production from some sets of perforations.

Ball sealers usually comprise small rubber-coated balls suspended in thetreating fluid and pumped into the well along with the treating fluid.The balls are carried down to the perforations communicating with thehigh permeability formation zone. The ball sealers seat on theseperforations and divert the treating fluid to a formation zone having alower permeability. In some cases, the presence of such ball sealers inthe wellbore after the treatment presents operational problems duringtheir retrieval. Use of degradable balls can help eliminate theseproblems, as reported in U.S. Pat. No. 6,380,138 issued to Ischy et al.Balls made of polyester polymer degrade with time, forming solubleoligomers and allowing perforations to re-open.

The fifth method is the Induced Stress Diversion Technique. This issimply an application of staged hydraulic fracturing treatments withoutthe use of any positive isolation, such as bridge plugs, frac baffles,sand plugs, or ball sealers. The ISD technique combines the advantagesof the Limited Entry and multi-staged fracturing techniques. Thetechnique involves pumping multiple fracs in a well and relying on theinduced stress imparted by an earlier fracture stimulation to divert thesubsequent fracture to the desired zone without positive zonalisolation. In this approach, the induced stress resulting from hydraulicfracturing of preceding stages functions as input energy to effectivelydivert the fracs to successive stages. The ISD procedure may be used toperforate and fracture multiple, discrete pay intervals by repeating theprocess as many times as needed (see FIG. 1). Some ISD techniques mayinclude methods to induce screenouts to help with the diversion.

However, ISDT requires good knowledge of reservoir properties. Thismakes ISDT not easily repeatable in areas with varying properties. Inorder to achieve maximum stress diversion, an optimized fracturingtreatment is required based on mechanical properties of the formation.This often necessitates acquiring data using a design tool, such as aDataFRAC™ (Trade name of Schlumberger Technology Corp.), and successiveredesigns of the approach. This takes time. In addition, redesign isstrongly dependent on critical assumptions about formation properties.As a result, currently there is no reliable methodology to justify theuse of ISDT in tight gas reservoirs. Therefore, there still exists aneed for easy and reliable methods for diversion, multi-stagefracturing, or temporary sealing in the downhole environment.

SUMMARY OF EMBODIMENTS OF THE INVENTION

A method for well treatment by forming a temporary plug in a fracture, aperforation, or a wellbore (or more than one of these locations)penetrating a subterranean formation is provided. A method of welltreatment in accordance with one embodiment of the invention includes:injecting a slurry comprising a degradable material, allowing thedegradable material to form a plug in a perforation, a fracture, or awellbore penetrating a formation; performing a downhole operation; andallowing the degradable material to at least partially degrade after aselected duration such that the plug disappears. The degradablematerials may be a polymer or co-polymer of esters, amides, or othermaterials. The degradable material degrades after a selected durationunder the downhole conditions such that no additional intervention isneeded to remove the plug.

The temporary blockage by plug formation allows other well operations tobe performed without damaging the existing fracture or withoutinterference from the existing fracture. Such other well operations mayinclude fracturing other zones, well repair, or installation of downholeequipment. As an example, a formation layer is fractured, the treatmentof the invention is performed, and another layer is fractured. Thesesteps may then be repeated again. Preferably, no sand plug, bridge plug,or any isolation device is used. Preferably any undegraded material isproduced with produced fluid without any need to assist in its removal.The treatment may be done such that the wellbore between the plug andthe wellhead is filled with fluid and the hydrostatic pressure on thewellbore side of the plug is greater than on the other side of the plug.

In other embodiments the slurry contains other particulates (such asproppants) or absorbents. Furthermore, other additives may be added toincrease or decrease the degradation rates of the degradable materials.Modeling techniques known in the art may be used with embodiments of theinvention to optimize the parameters for the well treatment. Forexample, modeling may be used to determine the duration needed for theoperation to be performed, and the degradable material, itsconcentration, and pumping rate are then selected accordingly. The mainlimitation of ISD is that the induced stress field is limited in theamount of stress it can create, typically in the range of 500 psi (3.44MPa) or so. If the zones' fracture pressures are greater than about 500psi (3.44 MPa), the differential stress created will not be enough toprevent the original fracture from taking subsequent injections.

Other aspects and advantages of the invention will become evident fromthe following description and the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustrating an induced stress diversiontechnique as used in the prior art.

FIG. 2 shows a chart illustrating excess pressure needed to fracture apay zone as a function of depth and spacing between pay zones.

FIG. 3 shows a graph illustrating excess pressure for 1500 psi (10.34MPa) in-situ stress differential between pay zone and shale.

FIG. 4 shows a chart illustrating the decomposition of plugs made ofpolylactic acid fibers having different molecular weights (as evidencedby a rapid increase of flow through the plugs) at 250° F. (121° C.) andat 1000 and 2500 psi (6.89 and 17.24 MPa), in accordance with oneembodiment of the invention.

FIG. 5 shows a chart illustrating the decomposition of plugs made ofdegradable materials and proppant, as evidenced by a rapid increase offlow through the plugs, in the presence of kerosene at 250° F. (121° C.)and 2500 psi (17.24 MPa), in accordance with one embodiment of theinvention.

FIG. 6 shows a schematic illustrating the bridging and blocking ofproppant in a fracture in accordance with one embodiment of theinvention.

FIG. 7A and FIG. 7B show a schematic illustrating plugging of aperforation in accordance with one embodiment of the invention.

FIG. 8A and FIG. 8B show a schematic illustrating plugging of a wellborein accordance with one embodiment of the invention.

FIG. 9A and FIG. 9B show a schematic illustrating plugging of a wellboreby using a degradable material and other materials (such as sand) inaccordance with one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention relate to methods for temporarily blockingwellbores, perforations, or formation fractures so that other work(e.g., fracturing of other zones, workover, well repair, installation ofdownhole equipment, etc.) can be performed more efficiently or withoutdamaging existing fractures. The temporary blocking is achieved by usingdegradable materials that will degrade within a desired period of time.As applied in fracturing, the techniques of the invention are similar tothe ISDT's that are currently used on U.S. land.

Degradable materials have been used for fluid loss control and fordiversion in the past. Examples include rock salt, graded rock salt,benzoic acid flakes, wax beads, wax buttons, oil-soluble resin material,etc. However, these materials have been used in sizes and shapesdesigned to build filter cakes on wellbore or fracture faces; they havenot been used to plug wellbores, perforations, or fractures.

Various degradable materials are used with embodiments of the invention.Such materials include inorganic fibers, for example of limestone orglass, but are more commonly polymers or co-polymers of esters, amides,or other similar materials. They may be partially hydrolyzed atnon-backbone locations. Examples include polyhdroxyalkanoates,polyamides, polycaprolactones, polyhydroxybutyrates,polyethyleneterephthalates, polyvinyl alcohols, polyvinyl acetate,partially hydrolyzed polyvinyl acetate, and copolymers of thesematerials. Polymers or co-polymers of esters, for example, includesubstituted and unsubstituted lactide, glycolide, polylactic acid, andpolyglycolic acid. Polymers or co-polymers of amides, for example, mayinclude polyacrylamides. Materials that dissolve at the appropriate timeunder the encountered conditions are also used, for example polyolscontaining three or more hydroxyl groups. Polyols useful in the presentinvention are polymeric polyols solubilizable upon heating, desalinationor a combination thereof, and consist essentially ofhydroxyl-substituted carbon atoms in a polymer chain spaced fromadjacent hydroxyl-substituted carbon atoms by at least one carbon atomin the polymer chain. In other words, the useful polyols are preferablyessentially free of adjacent hydroxyl substituents. In one embodiment,the polyols have a weight average molecular weight greater than 5000 upto 500,000 or more, and from 10,000 to 200,000 in another embodiment.The polyols may if desired be hydrophobically modified to furtherinhibit or delay solubilization, e.g. by including hydrocarbylsubstituents such as alkyl, aryl, alkaryl or aralkyl moieties and/orside chains having from 2 to 30 carbon atoms. The polyols may also bemodified to include carboxylic acid, thiol, paraffin, silane, sulfuricacid, acetoacetylate, polyethylene oxide, quaternary amine, or cationicmonomers. In one embodiment, the polyol is a substituted orunsubstituted polyvinyl alcohol that can be prepared by at least partialhydrolysis of a precursor polyvinyl material with ester substituents.Although it is normally not necessary, the degradation may be assistedor accelerated by a wash containing an appropriate dissolver or thatchanges the pH or salinity. The degradation may also be assisted by anincrease in temperature, for example when the treatment is performedbefore steamflooding. Below, when we use the term degradable, we includeall of these suitably dissolvable materials.

These materials are typically used at high concentrations (e.g., >60lbm/1,000 gal (>7.2 g/L)) in order to form temporary plugs or bridges.The concentrations can be lower if the fiber slurry can lose water,which concentrates the fibers. The maximum concentrations of thesematerials that can be used may be limited by the surface addition andblending equipment available.

These degradable or dissolvable materials may be in any shape: forexample, powder, particulates, beads, chips, or fibers. Preferredembodiments may use these materials in the form of fibers. The fibersmay have a length of about 2 to about 25 mm, preferably about 3 to about18 mm. Typically, the fibers have a denier of about 0.1 to about 20,preferably about 0.15 to about 6. The fibers preferably degrade underdownhole conditions in a duration that is suitable for the selectedoperation.

While methods of the invention may be used in fracturing, workover, orother types of operations, for clarity, the following description willuse hydraulic fracturing as an example to illustrate embodiments of theinvention. It will also be assumed, as an example, that sequentialfracturing starts at the bottom of a vertical well, or the distal end ofa horizontal well, and progresses toward the wellhead. Of course, othersequences are possible, depending upon the stress profile. One ofordinary skill in the art would appreciate that this is not intended tolimit the scope of the invention to hydraulic fracturing. Instead,methods of the invention may also be used in other operations, such astemporary plugging of fractures or wellbores.

Some embodiments of the invention relate to temporarily blocking ofalready-created fractures so that other zones may be fractured. Asapplied to multi-stage fracturing, at the tail end of a fracturingtreatment, a degradable or dissolvable material is pumped to temporarilyplug a completed fracture. The temporary plug locks the proppants in afracture, making them immobile and causing substantial stress increaseand diversion in lower zones by means of a significant net pressureincrease due to the high likelihood of proppant bridging with thedegradable materials. In accordance with an alternative method of theinvention, a degradable material that can create a temporary packer ispumped after the proppant stages to temporarily seal the fracture bysealing the perforations. In another alternative, the plug is formed inthe wellbore to seal the perforations leading to the fracture. In yetanother embodiment, a plug is formed in more than one of theselocations. With this system, the fracture is protected and successivefracturing treatments, usually further up the hole, can be performedwithout the need for wireline intervention. The degradable material willdissolve with time and unplug the fracture. The degradable material maybe of various properties, shapes and contents. The material decay ordisintegration may be chemically, temperature or mechanically driven.These methods may be performed with any suitable equipment known in theart, including coiled tubing (CT) that has been installed in the wellsfor jetting new perforations. These methods of the invention are similarto the ISDT's that are currently used on the U.S. land. However, thedegradable material assisted diversion (DMAD), in accordance withembodiments of the invention, can provide much higher and more reliablestress diversion.

Degradable materials have been used in other downhole operations, suchas disclosed in U.S. patent application Ser. No. 11/156,966 filed onJun. 20, 2005, by Willberg et al., entitled “Degradable Fiber Systemsfor Stimulation.” This application is assigned to the assignee of thepresent invention and is incorporated by reference in its entirety. Thedegradable materials that may be used in accordance with embodiments ofthe invention can be comprised of various chemical compositions as longas they can degrade within the desired time period under the downholeconditions, which may include temperatures as high as 350° F. (about180° C.) or more and pressures as high as 20,000 psi (137.9 MPa) ormore. As noted above, some embodiments of the invention relate todegradable material assisted diversion and other embodiments relate tosealing of existing fractures, perforations, or wellbores. The followingwill describe the desired properties of the degradable materials inaccordance with these purposes: as stress diversion agents or as sealersof perforations, fractures, or wellbores.

Degradable Materials as Diversion Agents

In stress diversion multi-stage fracturing, the stress should exceed thebreakdown pressure and the net pressure increase during the subsequentstages. Stress or pressure for diversion to subsequent fracturing stagesmay result from at least four mechanisms: (1) normal increases in stresswith depth; (2) pipe friction; (3) temporary supercharge from fracturing(net pressure); and (4) increased stress on proppants. Each of thesefactors will be analyzed in detail. (Another mechanism of stressdiversion that naturally occurs in any job using polymer as theviscosifier is concentration of the polymer gel due to fluid loss, butthis will not be discussed further.)

In-situ formation stress may arise from overburden stress, thermalstress, and tectonic strains. The common estimate for fracture pressureincrease is 0.62 psi/ft (14 kPa/m). This suggests that identical rockswith normal pressure gradient have 620 psi (4.27 MPa) of diverting powerper 1000 ft (304.8 m) of separation. However, in reality, fracturepressure distribution is not homogeneous (it is usually a function ofthe lithology and of the pore pressure of the layers in question) anddoes not vary linearly with depth. Therefore, additional energy oftenneeds to be added to the system in order to ensure that the ISDtechnique works consistently.

Pipe friction decreases the bottomhole pressure with depth. This has apositive effect on the Stress Diversion. However, even at high flowrates (hence, high friction), the friction pressure normally does notexceed 1000 psi/1000 ft (22.6 kPa/m). (In practice, the frictionpressure diversion at best (e.g., for very viscous fluids) will not behigher than 1000 psi per 1000 ft. (6.90 MPa per 304.8 m)) A recent trendis to use low polymer loading for fracturing fluids. Such fracturingfluids will have low viscosity, and hence, lower friction pressure. As aresult, friction pressure does not play an important role in a typicalfracturing job using the ISD techniques. (With the limited entrytechnique, the limited number of perforations creates an artificialfriction pressure, not due to the fluid, but rather due to the limitednumber of perforations.)

Fracturing supercharge comes from the need to overcome the breakdownpressure to initiate the fracturing process. Breakdown pressures aretypically 5-10% higher than the fracture extension pressure, which isabout the same as the closure stress. Typical values for fracturedifferential gradients are 0.1-0.2 psi/ft (2.26−4.53 kPa/m) at a deptharound 10,000 ft (3048 m). This implies that it is worthwhile to haveconsiderable excess net pressure in the first (lower) fracture toovercome the breakdown pressure of the upper zone. However, to beuseful, the supercharge pressure in the first fracture should not bereleased via flowback.

FIG. 2 shows a chart of excess pressures typically required to fracturea formation at different depths (curve 21 for 5000 ft (1524 m); curve 22for 6000 ft (1829 m); and curve 23 for 7000 ft (2134 m)). In this chart,it is assumed that the fracture gradient is 0.65 psi/ft (14.7 kPa/m),whereas the breakdown pressure gradient is 0.75 psi/ft (17 kPa/m). It isevident from FIG. 2 that the deeper the pay zones are, the higher therequired excess pressures are for a given spacing between pay sands. Forinstance, if the first fracture was generated at 7500 ft (2286 m) andthe next at 7000 ft (2134 m), the required excess pressure in the firstfracture is approximately 370 psi (2551 kPa) (see curve 23). In tightgas formations, the required excess pressure may be as high as 2,000 psi(13.8 MPa) at depths higher than 10,000 ft (3048 m). These data allassume identical lithology and pore pressures. Variations in each ofthese could affect the curves.

When pumping stops after fracturing, the fractures will close on theproppant that has entered the fractures. Common industry practice forestimating closure stress on proppant is to subtract flowing bottomholepressure from the estimated in-situ stress of the pay intervalfractured. However, is has been shown that closure stress on proppantcan be significantly higher than expected due to the influence of thebounding layers. See Schubarth et al., “Understanding Proppant ClosureStress,” SPE 37489, SPE Production Operations Symposium, Oklahoma City,Okla., U.S.A., Mar. 9-11, 1997. Normally, a fracture does not propagatewithin the targeted sand bordering the lower and upper shales where theclosure stress is higher. The stress differential between sand in thepay zone and sand at the boundaries may vary between 500 and 2500 psi(3.44 and 17.2 MPa). The thickness of a pay interval can vary from 20 to200 ft (6.1 to 61 m). Based on these values, it has been shown that theexcess closure pressure can vary from 300 psi (2.1 MPa) for a 200 ft (61m) thick pay sand to 1500 psi (10.34 MPa) for a 20 ft (6.1 m) thick paysand, assuming 1500 psi (10.34 MPa) in-situ pressure differential at thesand-shale boundaries (see FIG. 3). The excess stress at the sand-shaleboundaries may have a negative effect on the fracture conductivity, butit has a positive effect on stress diversion. It should be noted thatthis mechanism is largely independent of mechanisms 1-3 of the fourmechanisms listed above, because fracture closure does not occur untilthe fracture supercharge has completely dissipated through leak-off orflowback. Therefore, this mechanism can be considered separate from theother three mechanisms.

The interplay between the first three diversion mechanisms may beunderstood better by analyzing them in formulas. First, let's assumethat:

-   -   the first pay sand is fractured and has a temporary supercharge        of Δ₁ psi;    -   the supercharge Δ₁ is sufficient to divert the second stage;    -   there is a normal stress increase with depth of 0.65 psi/ft        (14.7 kPa/m); and    -   the friction pressure of the fracturing fluid is 500 psi/1000 ft        (11.3 kPa/m).    -   With these assumptions, the governing equations can be written        as:    -   For the first zone:        p _(s) +p _(HS1) −p _(fr1)≦σ_(min 1)+Δ₁  (1)    -   For the second zone:        p _(s) +p _(HS2) −p _(fr2)−σ_(min 2)=Δ₂  (2)    -   where p_(s) is the surface pressure, p_(HS1) is the hydrostatic        pressure for the i th—zone, p_(fr i) is the i th friction        pressure, σ_(min i) is the i th in-situ stress, and Δ₂ is the        net pressure in the second zone.

If one substitutes Eq. (2) into (1), one obtains:Δ₂ −p _(HS2) +p _(fr2)+σ_(min 2) +p _(HS1) −p _(fr1)≦σ_(min 1)+Δ₁  (3)orΔ₂ +Δp _(HS)≦Δ₁ +Δp _(fr)+Δσ_(min)  (4)

-   -   where Δp denotes p₁-p₂. The right-hand side of inequality (4)        describes the positive isolation mechanisms or induced stresses,        while the left-hand part is the required excess pressure. With        the assumptions listed above, Δp_(HS)=50 psi/100 ft (11.3        kPa/m), Δp_(fr)=50 psi/100 ft (11.3 kPa/m)., and Δσ_(min)=65        psi/100 ft (14.7 kPa/m). Substituting these numbers into        inequality (4), one obtains for a spacing of 500 ft (152.4 m):        Δ₁≧Δ₂−325 psi (2.24 kPa)  (5)

Inequality (5) indicates that if the net pressure in the first fractureis fully released (due to flowback or leak-off), then there is anexcessive pressure of 325 psi (2.24 kPa) to divert the next stage. Inlow permeable hard rocks, a typical value for the net pressure may varybetween 1000 and 2000 psi (6.9 and 13.8 MPa). This means that the ISDmargin of safety may easily be exceeded in the case of first stageflowback.

The above description illustrates that while ISD techniques may provideeffective diversion in certain formations, these techniques may not workin other formations. Unfortunately, short of measuring the in-situstress of every pay zone to be fractured, there is no reliable way topredict which formations are suitable for ISD techniques. Embodiments ofthe invention provide more reliable diversion methods by addingdegradable materials to enhance the net stress of the pay zone that wasjust fractured.

In accordance with embodiments of the invention, to achieve a greaternet pressure in the first fracture, high concentrations of specialdegradable materials are used at the tail ends of fracturing treatments.The degradable materials may be fibers, powders, or any other forms.Laboratory and field experiments have shown that at high concentrationsof fibers, the proppant-fiber slurry may bridge. As a result, the jobmay screen out. This will lead to a significant increase in the netpressure and to good near-wellbore proppant placement. Such a proceduremay be called a “tail screenout.” Studies have also shown that fiberbridging is a complicated phenomenon, which requires special modeling todesign such a job properly. U.S. patent application Ser. No. 10/214,817filed on Aug. 8, 2002, by Boney et al. discloses methods andcompositions designed to cause tip screenouts. This application isassigned to the assignee of the present invention and is incorporated byreference in its entirety.

On the other hand, if the first fracture supercharge is gone (e.g., dueto leak-off or flowback), the fourth mechanism of those listed above(i.e., the incremental stress on proppants) is activated. FIG. 3 showsthat the incremental stress on proppants may successfully substitute forthe surcharge pressures described above, if one fractures several thinpay zones simultaneously in the first stage of fracturing.

The above description shows that in tight gas formations, there are twooptions to ensure sufficient stress for diversion: (a) maintain high netsurcharge pressure in the first pay sand, i.e. prevent or minimizeflowback; or (b) rely on the high incremental stress on proppant, i.e.immediate flowback after the first stage. The second option (relying onincremental stress on proppants) would be advantageous, if one fracturesseveral thin pay zones with considerable in-situ stress differentialbetween sands and shales.

The above description also shows that high degradable materialconcentrations at the tail end of a treatment may be used to: (a)sustain proppants, (i.e. to reduce settling rate during and aftertreatments and to reduce proppant flowback); and (b) ensure larger netsurcharge pressure in the preceding stages.

Furthermore, the above description also shows that appropriate designingand laboratory experiments are desirable to ensure that the degradablematerial assisted diversion (DMAD) techniques in accordance withembodiments of the invention work properly. In addition to design andlaboratory experiments, modeling may also be used to design properparameters for the degradable material assisted diversion. Variousmodeling techniques are known in the art for designing various jobs indownhole environments, such as well stimulation, completion, etc. One ofordinary skill in the art would appreciate that any of such modelingtechniques may be used with the DMAD techniques of the invention.

Degradable Materials as Scalers

Some embodiments of the invention relate to the use of degradablematerials as sealers to temporarily block perforations, fractures, orwellbores such that other operations may be performed withoutinterference from or damage to the existing fractures. When thedegradable materials are used as sealers of wellbores, perforations, orfractures, all of the above-described mechanisms for diversion are alsoapplicable. In addition, high concentrations (e.g., >60 lbm/1,000 gal(>7.2 g/L)) of a degradable material, for example in fiber form, may bepumped after the proppant stages to temporarily seal the wellbore,perforations, or fractures with fiber networks and to divert thesubsequent stages.

A number of laboratory experiments on the generation and testing offiber plugs have been performed. The testing showed that plugs of 1 cmdiameter and 2 cm long can withstand 2500 psi (17.2 MPa) of pressure for2-4 hours at 250° F. (121° C.), depending on the plug composition. FIG.4 shows lifetimes of fiber plugs made of polylactic acids (PLA) havingdifferent molecular weights. As shown in FIG. 4, the higher molecularweight fiber plugs have longer lifetimes under the testing conditions(250° F. (121° C.); 1000 psi (6.9 MPa)). For example, the plug having apolymer with a molecular weight of 77,600 has a lifetime of severalhours, while plugs made of higher molecular weight polymers have longerlifetimes (up to 6 hours).

In addition to using higher molecular weight polymers, the lifetimes ofplugs may also be increased by using delay agents that protect thepolymers from degradation. For example, for PLA polymers, the maindegradation mechanism is hydrolysis. By adding a hydrophobic agent tothe polymer (or plug), for example as a coating, the rates of hydrolysiswill be reduced. As a result, the lifetimes of the polymers (hence, thelifetimes of the plugs) will be increased. Therefore, it is possible tocontrol the lifetimes of plugs to suit the purposes of the operations.

Some embodiments of the invention use degradable fiber plugs asdescribed above. Other embodiments of the invention use plugs that areformed of degradable fibers and another material, such as proppants,other particulates (such as sand), or degradable absorbents (such aspolyacrylic acid-co-acrylamide). The inclusion of an absorbent materialmay help fill pores inside a plug and make it stronger. The use ofappropriate RCP proppants or small grain size non-RCP proppants givesquite satisfactory results: RCP/fiber plugs are able to withstand apressure differential of 2500 psi (17.2 MPa) at 250° F. (121° C.) forseveral hours, as shown in FIG. 5.

In accordance with some embodiments of the invention, degradablematerials are used in combination with methods of increasing the solidcontent of a slurry using particle-size distribution technology. With aproperly chosen multi-modal distribution of particle sizes, smallerparticles fill the void spaces between larger ones, resulting in aslurry requiring less water. Typical distributions use two or threedistinct particle size ranges. This provides a slurry with improved flowproperties and excellent set properties such as permeability andstrength. Thus, some embodiments of the invention use proppants ofdifferent sizes instead of RCP's. With these embodiments, the proppantcompositions may be optimized to achieve sufficient conductivity of theplugs after the fibers are degraded.

With this approach (i.e., multi-modal particle size distribution),various combinations of temporary perforation sealers can be achievedwith excellent properties. Because degradable or dissolvable materials,such as a polylactic acid fiber, may be selected to be compatible withformation fluids and their downhole lifetimes can be easily varied(e.g., by adding delay agents to increase their lifetimes), thisapproach is very attractive in the DMAD technique.

In designing multiple fracturing jobs, careful attention should be paidto the following items:

-   -   1) The chemicals (degradable materials) may be sensitive to the        environment, so there may be dilution and precipitation issues.        The sealer preferably should survive in the formation or        wellbore for a sufficiently long duration (e.g., 3-6 hours). The        duration should be long enough for: (a) wireline services to        perforate the next pay sand; (b) subsequent fracturing        treatment(s) to be completed; and (c) the fracture to close on        the proppant before it completely settles, providing the best        fracture conductivity. In tight gas formations with low leakoff,        this may be an issue.    -   2) Degradable Material Sealers will allow no flowback. As a        result, the fracture will be supercharged for a much longer        period. This is good for diversion. However, in low leakoff        formations, the shut-in time may become too long, which may        result in proppant settling. In this case, flowback after the        degradable materials is broken may be used to aid in suspending        the proppant in the fracture.

Thus, a compromise between these two considerations should be made. Inaccordance with embodiments of the invention, the chemical lifetimes inthe wellbore and the fracture preferably are not shorter than 2-3 hours.On the other hand, their lifetimes preferably do not exceed a certainlimit to allow one to flow back formations with extremely low leakoff.This indicates that an appropriate selection of the sealer type andadditives is important.

Consider the Degradable Material Sealer mechanisms in more detail. Weassume:

-   -   the first pay sand is fractured and has a temporary supercharge        of Δ₁ psi;    -   the sealing ability of the material is p_(MS)=1000 psi (6.9        MPa);    -   the induced stress is enough to divert the subsequent stage;    -   there is a normal stress increase with depth of 0.65 psi/ft        (14.7 kPa/m);    -   fracturing fluid friction pressure is 500 psi/1000 ft (11.3        kPa/m); and    -   the hydrostatic pressure difference is 500 psi (3.45 MPa).    -   With these assumptions the governing equation (4) can be        rewritten as:        Δ₂ +Δp _(HS)≦Δ₁ +Δp _(fr)+Δσ_(min) +p _(MS)  (6)    -   For stage spacing of 500 ft this will give        Δ₁≧Δ₂−1325 psi (9.1 MPa)  (7)

Taking into account that the sealer provides no flowback, i.e. most ofsupercharge Δ₁ will stay in the first fracture, it becomes clear thatthe degradable material sealer may be an excellent diversion tool,providing excess pressure up to or higher than 2000 psi (13.8 MPa).

From the above description, it is apparent that good knowledge aboutformation and reservoir fluid properties is important to employ theDegradable Material Assisted Diversion (DMAD) techniques appropriatelyfor multiple fracturing treatments. The following parameters areimportant ones to consider in optimizing a DMAD job: in-situ stressprofile; in-situ stress differential between pay sand and shales;reservoir fluid composition and its compatibility with degradablematerial; and proppant sustaining in the fracture. Some of theseparameters may be available from downhole measurements, while others maynot be available. As noted above, embodiments of the invention may use amodeling technique to optimize the DMAD job. Any parameters notavailable may be optimized using a suitable modeling method known in theart.

As illustrated in the above description, embodiments of the inventionuse degradable materials to block a perforation, fracture, or wellboretemporarily so that work may be performed in other zones. In accordancewith some embodiments of the invention, at the tail end of a fracturingtreatment, a degradable material is pumped at a high concentration totemporarily plug a completed fracture, and to lock the proppant in afracture making it immobile and causing substantial stress increase anddiversion from lower zones by means of a significant net pressureincrease due to a higher likelihood of proppant bridging. In accordancewith some embodiments of the invention, a degradable material that cancreate a temporary packer is pumped after the proppant stages totemporarily seal the perforations associated with the fracture, or totemporarily seal the wellbore adjacent to these perforations. With thissystem, the fracture is protected and a subsequent fracturing treatmentfurther up the hole may be performed without the need for wirelineintervention.

Degradable materials will dissolve with time and unplug the fracture.The degradable materials may be of a variety of properties, shapes andcompositions. The material decay or disintegration may be chemically,temperature or mechanically driven. Methods of the invention may beperformed with any suitable equipment used in the industry, such ascoiled tubing that is installed in the well ready to jet newperforations. While methods of the invention are conceptually similar toISDT, the degradable material assisted diversion (DMAD) techniques ofthe invention guarantee much higher and more reliable stress diversion.

FIG. 6 shows a schematic representation of a method in accordance withsome embodiments of the invention. In accordance with this method, adegradable material/chemical is added at the tail end of the proppantstages to induce proppant bridging. The addition of the materialsubstantially increases the likelihood of proppant bridging in thefracture created in an earlier stage (shown as pay zone 1). Onceproppant bridging occurs, proppants are locked in the fractures, whichprevents flowback and helps retain the supercharge stress in pay zone 1(shown as a dashed line pressure profile to the right). Proppantbridging may also induce a near-wellbore screen-out, which in turn cansubstantially increase the fracture supercharge in pay zone 1. Thesupercharge stress in pay zone 1 will divert the fracture fluids to thenext pay zone to be fractured (shows as pay zone 2).

As noted above, the added degradable materials may have various shapes(e.g., particulates or fibers). The sizes of the additives preferablyshould be selected based on the fracture width; they should be less thanthe fracture openings so that they can enter the fractures. In addition,the degradable material should withstand formation conditions for adesired duration (such as 3-6 hours), i.e., the compatibility demandsshould be met.

FIGS. 7A and 7B show another method in accordance with one embodiment ofthe invention. In this method, a degradable material is pumped at highconcentration after the proppant stages. The chemical along withproppant clogs up the perforations, creating a temporary seal. As shownabove, a temporary seal (or plug) formed of a degradable material maywithstand over 2500 psi (17.2 MPa) pressure differential at 250° F.(121° C.) for several hours depending on the formation conditions (seeFIGS. 4-6). In preferred embodiments, the seal or plug consists of oneor more degradable materials only. However, since there exists a risk ofover-pumping the proppant into the fracture, in some cases, it may beworthwhile to induce a tail screenout by adding a special chemical ormaterial (a bridging inducing material, such as glass fibers) at the endof proppant stages. The sealing and bridging inducing materials may bedifferent from the degradable materials. In accordance with theseembodiments of the invention, both materials should be compatible withreservoir fluids for the desired durations.

FIGS. 8A and 8B show another method in accordance with one embodiment ofthe invention. In accordance with this method, a temporary bridge plugmay be formed in the wellbore. The temporary bridge is formed of adegradable material. This method extends the sealing to the wellbore inorder to make sure that a preceding fracture is protected fromre-fracturing (re-opening an existing fracture). Similar to the methoddepicted in FIGS. 7A and 7B, a tail screenout may be induced. Althoughthere might exist a certain amount of sand in the perforations andwellbore, due to inducement of a tail screenout, the amount of sand inthe wellbore will be much less than using the Sand Plug technique. Infact, all the combinations and permutations of partially or completelyplugging wellbores, perforations, and fractures are embodiments of theinvention.

FIGS. 9A and 9B show another method in accordance with one embodiment ofthe invention. In this method, a degradable material is pumped togetherwith proppant at the tail end of a fracturing treatment to create acomposite plug in the perforations and/or wellbore. No induced tailscreenout is needed. In this case, the best stage diversion may beachieved. The degradable materials should be selected such that theywill survive for several hours in the wellbore. A possible disadvantageof the embodiment is the sand production from the material in thewellbore during flowback when the sealing material is gone.

As noted above, methods of the invention that form temporary bridges orseals in the perforations, fracture(s), wellbore, or any combination ofthese are used for subsequent fracturing or for other operations to beperformed downhole. In accordance with some embodiments of theinvention, after the temporary seal is formed, the well may undergovarious treatments instead of subsequent fracturing. For example, thewellbore may be repaired (acid treatments), or installation of anelectric submersible pump (ESP) may be performed. The plugging agent canbe selected to last sufficiently long to protect the formation over theexpected time period of the subsequent downhole operation.

Therefore, in accordance with some embodiments of the invention, afracture is temporarily sealed or blocked with a degradable material.The degradable material is used to temporarily protect the fracture frompost-job workover fluid damage, or to temporarily protect downholeequipment from fracture flowback damage. The selection of the degradablematerials depends on the expected damage, bottomhole conditions, and thedurations needed for protection.

In accordance with embodiments of the invention, degradable materialsare preferably compatible with different pH fracturing fluids and withbrines containing different concentrations of salts (such as sodiumchloride NaCl, calcium chloride CaCl₂, sodium bromide NaBr, potassiumchloride KCl, and others). The degradable materials should be compatiblewith temperature ranges as wide as possible. It is preferred that thedegradable materials are compatible with temperatures greater than 32°F. (0° C.). Degradable materials should be compatible with weightedbrines or completion fluids as well.

In accordance with some embodiments of the invention, different types ofchemicals may be pumped to accelerate or delay degradable materialdecomposition. Examples of delay agents may include any type ofhydrophobic material (for example, kerosene, oil, diesel, polymers,surfactants, etc), which will cover the surfaces of the degradablematerials to slow their interactions with water. For polyols, forexample, such as partially hydrolyzed polyvinyl acetate, for example,salts may be included in the fluid; high ionic strength decreases thesolubility of such materials. Examples of accelerator agents may includeany high or low pH liquids (for example caustic or acid solutions),which will accelerate the decomposition of the degradable materials.

As noted above, methods of the invention for diversion or sealing offormation fractures using degradable materials may be based on resultsobtained from modeling. One of ordinary skill in the art wouldappreciate that various formation modeling techniques are available forhydraulic fracturing, such as Schlumberger's FracCADE Stimulator™ andthe methods disclosed in U.S. Pat. No. 6,876,959 issued to Pierce etal., which is assigned to the assignee of the present invention and ishereby incorporated by reference in its entirely. Other availablesoftware, for example, includes pseudo three-dimensional (P3D) hydraulicfracture simulators and planar three-dimensional (PL3D) hydraulicsimulators (including GOHFER™ from Stim-Lab and Marathon Oil Co.).Embodiments of the invention are not limited to any particular modelingmethod.

In accordance with some embodiments of the invention, modeling is usedto simulate induced stress diversion for the formation of interest.Then, the types and amounts of fluids to be used, and the durations andpumping rates for the fracturing job are accordingly selected.

Embodiments of the invention provide efficient methods for divertingstress/pressures for staged fracturing. One of ordinary skill in the artwould appreciate that these methods may be applied in any type of well,including vertical, deviated or horizontal wells, and open or casedhole.

While the above description uses hydraulic fracturing to illustrateembodiments of the invention, one of ordinary skill in the art wouldappreciate that methods of the invention may also be used in other typesof fracturing, including slickwater (or waterfrac) and acid fracturing.One of ordinary skill in the art would appreciate that various acidfracturing methods may be used with embodiments of the invention,including methods of generating acid downhole (using an emulsified acid,encapsulated acid, or solid acid precursor). For example, U.S. patentapplication Ser. No. 10/605,784 filed on Oct. 27, 2003, by Still et al.discloses the use of solid acid precursors to provide controlled releaseof acid by hydrolysis or dissolution. This application is assigned tothe assignee of the present invention and is incorporated by referencein its entirety.

In accordance with one embodiment of the invention, the degradablematerials may be used in acid fracturing. The degradable materials formtemporary blocks in the high permeability zones to divert the acid fracto the zones in need of treatments. The acid fracturing may use a solidacid precursor, for example. The solid acid precursor may be lactide,glycolide, polylactic acid, polyglycolic acid, a copolymer of polyaceticacid and polyglycolic acid, a copolymer of glycolic acid with otherhydroxyl-, carboxylic acid-, or hydroxycarboxylic acid-containingmoieties, a copolymer of lactic acid with other hydroxyl-, carboxylicacid-, or hydroxycarboxylic acid-containing moieties, or mixture of thepreceding. The solid acid may be mixed with a second solid that reactswith an acid to increase the rate of dissolution and hydrolysis of thesolid acid precursor.

Furthermore, embodiments of the invention may also be used totemporarily plug the fractures or wellbore in order to achieve thedesired effects or to perform other work. For example, methods of theinvention may be used to temporarily shut in the well after fracturingso that the fractures can relax. For such a purpose, the duration istypically short, about 0.5 hour, for example. One can select a properdegradable material to achieve the desired duration. The degradableplugs in accordance with embodiments of the invention may also be usedas “kill pills” to temporarily plug perforations or fractures.

The addition of the degradable materials in accordance with embodimentsof the invention may be practiced with existing equipment. One ofordinary skill in the art would appreciate that various methods used inthe field may be adapted for use with methods of the invention. Forexample, the degradable materials may be mixed with proppants inblenders. The addition of the chemicals (degradable materials or otheradditives) may be managed by means of a modified feeder or a flush kit.Alternatively, it is also possible to place the degradable materials bymeans of coiled tubing in the wellbore. Similarly, it is also possibleto use coiled tubing for the injection (addition) of delay oraccelerator agents. It is also possible to place the degradablematerials via coiled tubing or tubing while simultaneously fracturingdown the annulus between the coiled tubing and the casing. Thedegradable materials would mix with proppant or simply follow theproppant in the casing to cause the bridging.

The methods of the invention may also be combined with methods of usingfibers to assist in the transport of proppant, for example in slickwatertreatments, for example as described in U.S. patent application Ser. No.11/156,966, entitled “Degradable Fiber Systems For Stimulation”, filedJun. 20, 2005, assigned to the same assignee as the present application,and hereby incorporated in its entirety. The methods may also be used intreatments in which fibers are also used in proppant-free fluids such asin the pads of fracturing treatments, or in prevention of fluid lossinto natural fractures, for example as described in U.S. patentapplication Ser. No. 11/206,898, entitled “Methods For Controlling FluidLoss,” filed Aug. 18, 2005, assigned to the same assignee as the presentapplication, and hereby incorporated in its entirety. Preferably, thesame fiber is used in all stages of these combination treatments. As anexample, the same degradable fiber is used in the pad of a fracturingtreatment stage, and/or in the main fracturing fluid of the stage toassist proppant transport, and at the end of the stage for degradablematerial assisted diversion.

It should be noted that the pumping rate may be reduced at the end of afracturing stage to promote screenout, for example of fibers andproppant in hydraulic fracturing or of fibers in acid fracturing. Itshould also be noted that the first fracture will be placed in theweakest part of the formation, which could be at the wellhead end, thefar end, or anywhere in between, and the layers could be fractured inany sequence. If one or more plugs are in the wellbore rather than inthe fractures, this would require removing one or more plugs during thetreatment.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method of well treatment, comprising: a) injecting a slurrycomprising a degradable material selected from the group consisting ofpowder, beads and chips, and an additive for accelerating degradation ofthe degradable material; b) allowing the degradable material to form aplug in one or more than one of a perforation, a fracture, and awellbore in a well penetrating a formation; c) performing a downholeoperation; and d) allowing the degradable material to at least partiallydegrade after a selected duration such that the plug disappears.
 2. Themethod of claim 1, wherein the degradable material is selected from apolymer of lactide, glycolide, polylactic acid, polyglycolic acid,amide, and mixtures thereof.
 3. The method of claim 2, wherein the welltreatment comprises chemical strimulation.
 4. The method of claim 1,wherein the degradable material is present at a concentration of no lessthan 40 lbm/1,000 gal (4.8 g/L).
 5. The method of claim 1, wherein theoperation is a fracturing operation and the method further comprisesinjecting a second material to induce a screenout during the fracturingoperation.
 6. The method of claim 1, wherein the slurry furthercomprises a particulate material.
 7. The method of claim 6, wherein theparticulate material is degradable.
 8. The method of claim 6, whereinthe particulate material is a proppant.
 9. The method of claim 8,wherein the proppant comprises particles having a distribution of sizes.10. The method of claim 1, wherein the slurry further comprises anadditive for delaying degradation of the degradable material.
 11. Themethod of claim 1, further comprising modeling to optimize parametersfor the well treatment, wherein the step of injecting the slurry isbased on the optimized parameters.
 12. The method of claim 11, whereinthe optimized parameters comprise the selected duration before thedegradable material degrades.
 13. The method of claim 1, wherein thewell treatment comprises hydraulic fracturing.
 14. The method of claim13, wherein the hydraulic fracturing comprises injecting a pad, said padcomprising the degradable material.
 15. The method of claim 13, whereinhydraulic fracturing is applied to more than one layer of a multilayerformation.
 16. The method of claim 15, wherein the step of injecting theslurry is performed at the tail end of fracturing a layer.
 17. Themethod of claim 16, wherein no bridge plug or sand plug is placed in thewellbore before fracturing a subsequent layer.
 18. The method of claim16, wherein the slurry further comprises a proppant, wherein thedegradable material forms the plug in the fracture by bridging with theproppant when placed in the fracture.
 19. The method of claim 1, whereinthe well treatment comprises acid fracturing or waterfrac.
 20. Themethod of claim 1, wherein the selected duration for the plug todisappear is between 0.5 and 6 hours.
 21. The method of claim 1, whereinthe step of injecting the slurry is via coiled tubing installed in thewell.
 22. The method of claim 1 wherein the degradable materialcomprises a polyvinyl ester.
 23. The method of claim 1, wherein thetreatment is used to isolate a region selected from the fracture, aproduction zone, and a part of the wellbore, wherein the part of thewellbore is above the formation pressure.
 24. The method of claim 1,wherein the degradable material is selected from limestone materials andglass materials.
 25. A method of well treatment, comprising: a)injecting a slurry comprising a degradable material selected from thegroup consisting of powder, beads, chips, fibers, or any combinationthereof, and an additive for accelerating degradation of the degradablematerial; b) allowing the degradable material to form a plug in one ormore than one of a perforation, a fracture, and a wellbore in a wellpenetrating a formation; and, c) performing a downhole operation. 26.The method of claim 25, wherein the well treatment further comprisesallowing the degradable material to at least partially degrade after aselected duration such that the plug disappears.
 27. A method of welltreatment, comprising: a) injecting a slurry comprising a degradablematerial selected from the group consisting of powder, beads, chips, andan additive for accelerating degradation of the degradable material; b)allowing the degradable material to form a plug in one or more than oneof a perforation, a fracture, and a wellbore in a well penetrating aformation; and, c) performing at least one downhole operation.